U.S. patent number 4,519,695 [Application Number 06/464,455] was granted by the patent office on 1985-05-28 for image density control method for electrophotography.
This patent grant is currently assigned to Ricoh Company, Ltd.. Invention is credited to Yutaka Hasegawa, Kazuo Murai.
United States Patent |
4,519,695 |
Murai , et al. |
May 28, 1985 |
Image density control method for electrophotography
Abstract
A method of controlling an image density in electrophotography
by controlling at least one of various image density parameters in
response to values detected from different pattern areas, the image
density parameters including an amount of charge deposited on a
photoconductive element by a charger, bias voltage for development,
toner concentration in a developer, amount of toner supply to a
developing unit and image transfer potential. The method forms at
least two pattern areas having different potentials on the surface
of the photoconductive element by at least one of various means for
forming charge patterns which include controlling the energization
of the charger, controlling an illumination lamp and projecting an
image pattern. In different ranges respectively assigned to the two
pattern areas, there is digitized at least one of values associated
with an image density which include a surface potential of the
pattern area before development, toner density of the pattern area
after development, surface potential of the pattern area after
development and image density of an area of a transferred image
which corresponds to the pattern area. The digitized data of the
different patterns are compared with each other.
Inventors: |
Murai; Kazuo (Tokyo,
JP), Hasegawa; Yutaka (Tokyo, JP) |
Assignee: |
Ricoh Company, Ltd. (Tokyo,
JP)
|
Family
ID: |
12035322 |
Appl.
No.: |
06/464,455 |
Filed: |
February 7, 1983 |
Foreign Application Priority Data
|
|
|
|
|
Feb 12, 1982 [JP] |
|
|
57-20731 |
|
Current U.S.
Class: |
399/60; 118/691;
355/77 |
Current CPC
Class: |
G03G
15/00 (20130101); G03G 15/0855 (20130101); G03G
15/065 (20130101); G03G 15/5041 (20130101) |
Current International
Class: |
G03G
15/06 (20060101); G03G 15/08 (20060101); G03G
15/00 (20060101); G03G 015/00 () |
Field of
Search: |
;355/14R,14D,14CH,77,14E,14C ;118/679,665,689-691,663,664 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Moses; Richard L.
Attorney, Agent or Firm: Alexander; David G.
Claims
What is claimed is:
1. A method of controlling an image density in electrophotography
by controlling at least one of various image density parameters in
response to values detected from different pattern areas, the image
density parameters including an amount of charge deposited on a
photoconductive element by a charger, bias voltage for development,
toner concentration in a developer, rate at which toner is supplied
to the developing unit and image transfer potential, comprising the
steps of:
(a) forming at least two pattern areas having different potentials
on the surface of the photoconductive element by at least one of
various means for forming charge patterns which include controlling
the energization of the charger, controlling an illuminating lamp
and projecting an image pattern;
(b) producing in digitized form in different ranges respectively
assigned to the two pattern areas, at least one of values
associated with an image density which include a surface potential
of the pattern area before development, toner density of the
pattern area after development, surface potential of the pattern
area after development and image density of an area of a
transferred image which corresponds to the pattern area;
(c) comparing the digitized data of the different patterns and,
based on the result of the comparison, setting up correspondence of
the values associated with image density to a relation in magnitude
between the digitized data and;
(d) controlling said at least one image density parameter in
accordance with a predetermined function based on the result of
said comparison performed in step (c).
2. A method as claimed in claim 1, in which the charge patterns are
formed by projecting black and white image patterns having a
substantial density difference onto the surface of the
photoconductive element.
3. A method as claimed in claim 1, in which the value associated
with image density is a toner density on the surface of the
photoconductive element, said toner density being sensed by a
photosensor.
4. A method as claimed in claim 3, in which the image density
parameter is a rate at which toner is supplied to a developing
station.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method of controlling the
density of images reproduced by an electrophotographic process and,
more particularly, to an image density control method which forms
at least two test patterns on a photoconductive element which have
greatly different latent image potentials, detects values
associated with the image densities of the two patterns or of toner
images corresponding to the two patterns, and controls an image
density in response to the detected values.
In an electrophotographic or electrostatic recording apparatus, a
latent image is formed electrostatically on a photoconductive
element by a predetermined procedure and the latent image is
developed by fine particles of colored toner supplied from a
developing unit. Usually, the toner is charged to a polarity
opposite to that of the latent image so that it may be
electrostatically deposited on the latent image.
A method available for so charging a toner relative to a latent
image employs a developer constituted by a toner and a carrier and
stirs them together for frictional charging. This type of developer
is usually referred to as a two-component developer. While the
developing method using the two-component developer is capable of
sufficiently charging a toner to a desired polarity, it requires
adequate means for maintaining a constant toner concentration in
the developer because only the toner is consumed by the
development. It is therefore necessary to measure the varying toner
concentration in the developer.
For the measurement of a toner concentration, a somewhat indirect
method is known as disclosed in Japanese Patent Publication No.
16199/68. This method comprises the steps of forming a reference
latent image pattern electrostatically on a photoconductive drum,
developing the reference pattern and photoelectrically measuring
the density of the developed image. In a direct method heretofore
proposed, on the other hand, the weight or permeability of a
developer is measured. Other known methods include one which
controls a toner density by detecting a surface potential of a
toner image on a photoconductive element (Japanese Patent Laid-Open
Publication No. 92138/78). Various other methods have also been
proposed for general image density control purpose such as one
which controls the bias voltage for development in accordance with
a difference in reflectivity between a reference density plate and
an original document (Japanese Patent Laid-Open Publication No.
103736/78), one which controls the developing characteristics by
detecting an image density during a copying cycle which uses a
reference original document (Japanese Patent Laid-Open Publication
No. 141645/79), and one which controls the amount of charge on a
photoconductive element, bias voltage for development and/or
illumination intensity by detecting an image density on an original
document, latent image potential and toner image density (U.S. Pat.
No. 2,956,487).
One of these known image density control methods employs light and
dark patterns, such as black and white patterns, which are
electrostatically formed on a photoconductive element. A problem
has existed in this type of method in that where the black and
white latent patterns on the photoconductive element are developed
and the resulting toner densities of the two patterns are sensed by
a photosensor, the toner tends to smear the surfaces of light
emitting and light receiving elements which constitute the sensor
in combination. This, coupled with the deterioration of the
coactive elements, effects the input/output characteristics of the
sensor so that errors are introduced into the result of
measurement.
U.S. Pat. No. 4,082,445 discloses a method which measures a toner
density while compensating for the variation in the characteristics
of a light receiving element, surface of a photosensitive element
and the like due, for example, to a change in the power source
voltage, toner deposition, temperature variation and deterioration
due to passage of time. In this prior art method, a non-image area
on a photoconductive element where the toner is absent is
photoelectrically detected first. Because the surface of a
photoconductive element has a predetermined reflective power
(reflectivity), periodic detection of such a non-image area is
effective to see a change in the characteristics of the light
receiving element. The change is compensated for by increasing the
current which flows through the light emitting element, until the
output of the light receiving element returns to a normal value.
After the light receiving element has regained its normal
density/output voltage characteristic, a density of a reference
image is measured to control the toner density. This method,
however, invites a disproportionate increase in cost due to the
need for an additional circuit for increasing the current supply to
the light emitting element. Moreover, the life of the light
emitting element becomes shorter owing to the increased load acting
thereon.
These drawbacks may be overcome by forming at least two test
patterns on a photoconductive element, digitizing values associated
with image densities of the test patterns by different resolutions,
computing a ratio of the values associated with the image densities
in terms of the digital data, and controlling various parameters
related with the image densities in correspondence with the
computed ratio, as disclosed in Japanese patent application No.
56-178891/81. The different resolutions assigned to the discrete
patterns allow the values associated with the image densities to
accurately represent the values of the discrete patterns, while the
computation of a ratio equally weights the values associated with
the respective image densities. Thus, the difference in resolution
is equivalent to multiplication or division by a predetermined
value. Various parameters related with image densities, therefore,
accurately reflect the values associated with the image densities
of the different patterns. Furthermore, any fluctuation in the
values associated with the image densities attributable to a change
in the characteristics of the sensor and photoconductive element
appear proportionally in the different patterns, so that the
control based on the ratio promotes a stable density control
against the variation in characteristic.
Such a method as disclosed in Japanese patent application No.
56-178891/81 still involves a problem due to the use of a
microprocessor for computing the ratio (division). As well known in
the art, division by a microprocessor requires an intricate
computing program and slows down the operation.
The present invention contemplates to omit the division to simplify
the computing program although controlling the image density on the
basis of the density ratio of at least two test patterns as in the
prior art method discussed above. The simpler program will make the
construction of a control device simpler and the processing
faster.
Suppose that the image density parameter is a supplementary supply
of toner, and that a white pattern on a photoconductive element has
a developed image density (more strictly, its reciprocal) V.sub.SG
while a black pattern has a developed image density (more strictly,
its reciprocal) V.sub.SP. Then, in a preferred embodiment, the
toner supply is needless when the ratio V.sub.SP /V.sub.SG is
smaller than 4/1 due to a sufficient contrast, but necessary when
otherwise due to an insufficient contrast. Employing such a
threshold value (4/1) and, therefore, V.sub.SP =4V.sub.SG from
V.sub.SP /V.sub.SG =4/1, the density can be controlled such that
the toner should be supplied when V.sub.SP .gtoreq.4V.sub.SG due to
a short contrast but not when V.sub.SP <4V.sub.SG due to an
adequate contrast. Thus, whether or not the toner supply is needed
can be determined without resorting to division. Meanwhile, when
analog signals V.sub.SPa and V.sub.SGa indicative of detected
densities are respectively digitized at the ranges of 1:4, the
digital data of V.sub. SPa represents V.sub.SP and the digital data
of V.sub.SGa, 4V.sub.SG. This permits whether or not to supply the
toner to be determined merely by comparing the digital data of
V.sub.SPa and V.sub.SGa.
In light of this, the present invention selectively supplies a
toner by converting analog data associated with black and white
patterns into digital data each in a predetermined range which is
different from the other. The ranges can be easily determined by
dividing an input analog signal to an A/D converter by a resistance
type potential divider.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to efficiently
control the image density in electrophotography.
It is another object of the present invention to provide an image
density control method for electrophotography which controls the
image density on the basis of a ratio in image density between at
least two test patterns formed on a photoconductive element.
It is another object of the present invention to provide an image
density control method for electrophotography which quickly
controls the image density by a simple control device and
computation.
It is another object of the present invention to provide a
generally improved image density control method for
electrophotography.
A method of controlling an image density in electrophotography
embodying the present invention controls at least one of various
image density parameters in response to values detected from
different pattern areas, the image density parameters including an
amount of charge deposited on a photoconductive element by a
charger, bias voltage for development, toner concentration in a
developer, amount of toner supply to a developing unit and image
transfer potential. The method forms at least two pattern areas
having different potentials on the surface of the photoconductive
element by at least one of various means for forming charge
patterns which include controlling the energization of the charger,
controlling an illumination lamp and projecting an image pattern.
In different ranges respectively assigned to the two pattern areas,
there is digitized at least one of values associated with an image
density which includes a surface potential of the pattern area
before development, toner density of the pattern area after
development, surface potential of the pattern area after
development and image density of an area of a transferred image
which corresponds to the pattern area. The digitized data of the
different patterns are compared with each other.
Other objects and features, together with the foregoing, are
attained in the embodiments described in the following description
and illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a copying machine furnished with a
toner density control apparatus to which the present invention is
applied;
FIGS. 2, 2A, 2B are a circuit diagram showing electric connection
between a microprocessor and an A/D converter included in the
machine of FIG. 1;
FIG. 3 is a block diagram showing details of the A/D converter
indicated in FIG. 2;
FIGS. 4, 4A, 4B are a demonstrating the operations of a copy
control microprocessor for the control of constant amount toner
supply and the control of toner density control command timing;
FIGS. 5a, 5a-1, 5a-2, 5a-3, are a flowchart demonstrating interrupt
control of an image density control microprocessor;
FIGS. 5b, 5b-1, 5b-2, 5b-3, 5b-4 are a flowchart showing a main
flow;
FIG. 6 is a graph showing a relationship between output voltages of
a toner density sensor and toner image densities; and
FIG. 7 is a circuit diagram of an A/D converter applicable to
another embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the image density control method for electrophotography of
the present invention is susceptible of numerous physical
embodiments, depending upon the environment and requirements of
use, substantial numbers of the herein shown and described
embodiments have been made, tested and used, and all have performed
in an eminently satisfactory manner.
Referring to FIG. 1 of the drawings, a copying machine to which the
method of the present invention is applicable is shown. The copying
machine includes a glass platen 10 on which an original document
(not shown) is laid. Image light from the document is projected
onto a photoconductive drum 12 by an imaging system which is made
up of a first mirror 14, second mirror 16, in-mirror lens 18 and
third mirror 20. The drum 12 is rotated counterclockwise as
indicated by an arrow while the first and second mirrors 14 and 16
are moved to the left in synchronism with the rotation of the drum
12 and at a predetermined velocity ratio thereto. A latent image
electrostatically formed on the drum 12 is developed by a developer
26 which is supplied by a developing roller 24 of a developing
unit. The resulting toner image on the drum 12 is transferred onto
a sheet of paper (not shown) by a transfer charger 28. The sheet
carrying the toner image thereon is fed to a fixing station by a
belt 30.
To practice the method of the present invention in such a copier, a
white optical mark MR.sub.G is carried by an edge portion of the
glass platen 10 in an image projection field and in a position
corresponding to a home position of the first mirror 14. A black
optical mark MR.sub.P is also carried by the glass platen 10 to the
left of the white optical mark MR.sub.G. As the first mirror 14
strokes to the left for scanning the document on the glass platen
10, patterns P.sub.G and P.sub.P of the white and black marks are
electrostatically and successively formed on the drum 12 as latent
images. A photosensor 32 is located between the developing unit
(roller 24) and the transfer charger 28 to sense a toner density on
the surface of the drum 12. The output signal of the photosensor 32
is fed to an amplifier 34 to be amplified and wave-shaped thereby.
The output of the amplifier 34 is coupled to an analog-to-digital
or A/D converter 36 and the digital output of the latter is fed to
a microprocessor 38. The microprocessor 38 computes the density
ratio between the toner image (toner image pattern) corresponding
to the white pattern P.sub.G and that corresponding to the black
pattern P.sub.P, thereby determining an amount of toner to be
supplied to the developing roller 24. For a period of time matching
with the specific amount of toner supply, the microprocessor 38
supplies a solenoid driver 40 with a solenoid drive command so that
the driver 40 energizes a clutch solenoid 42 for the duration of
the solenoid drive command. Upon the energization of the solenoid
42, a roller 44 associated with a hopper 46 is coupled to a drive
system for the drum 12 and thereby rotated to supply the desired
amount of toner from the hopper 46.
In FIG. 1, also arranged around the drum 12 are a main charger 48
for uniformly charging the drum surface, and an erase lamp 50 for
removing the charge from those areas of the drum surface just ahead
and past of the image and outside a sheet size. A second
microprocessor 52 is included in the illustrated copier to control
copying operations other than the toner supply performed by the
microprocessor 38. Because the copier employs an amount of toner
supply which is predetermined in accordance with a sheet size or
format, the microprocessor 38 functions to supply an amount of
toner supplementary to the constant toner supply for each copy.
Referring to FIG. 2, the electric connection of the copier shown in
FIG. 1 is illustrated in detail. The photosensor 32 comprises a
light emitting diode or LED 32a and a phototransistor 32b. Light
emitted from the LED 32a is reflected by the drum 12 and becomes
incident on the phototransistor 32b. The emitter voltage of the
phototransistor 32b is fed directly to an input channel A.sub.1 of
the A/D converter 36 (MB4025 manufactured by Fujitsu Limited,
Japan) and, through voltage dividing terminals EX.sub.2 and
EX.sub.1, to an input channel A.sub.0. A digital (serial) data
output terminal DATA OUT of the A/D converter 36 is connected to an
interrupt terminal T.sub.1 of the microprocessor 38, while control
input terminals (A/D CLK-RS) thereof are connected to output ports
P24-P27 of the microprocessor 38.
The internal structure of the A/D converter 36 is shown in FIG. 3.
The A/D converter 36 is of the 8-bit A/D conversion type which can
be selectively supplied with input voltages V.sub.cc /2 and
V.sub.cc /8 by range selection, while being capable of expanding
the range to four times by range expansion. In a preliminary
experiment, the toner density gave a voltage V.sub.SG of 4.0 V in a
drum area corresponding to the white pattern P.sub.G (background
level), and a voltage V.sub.SP of 1.6 V in a drum area
corresponding to the black pattern P.sub.P (black level). For these
voltage levels, the maximum voltage at the input channels A.sub.0
-A.sub.3 is 2.5 V.
Based on the data mentioned above, a measurement range of 0-10 V
obtained by range expansion, V.sub.22 /2.times.4, is used for the
background level V.sub.SG and that of 0-2.5 V, V.sub.cc /2, for the
black level V.sub.SP. The emitter of the phototransistor 32b is
connected to the terminal EX.sub.2 of the A/D converter 36 while
the terminal EX.sub.1 is connected to the input channel A.sub.0.
Therefore, in A/D conversion with the input channel A.sub.0
designated, the range is expanded four times as 2.5/(7.5+2.5)=1/4;
thus, the input channel A.sub.0 is employed for the detection of
the background level V.sub.SG. The emitter of the phototransistor
32b is directly connected to the input channel A.sub.1 and,
therefore, the input channel A.sub.1 is employed for the detection
of the black level V.sub.SP. It follows that the digitized data of
the background level V.sub.SG multiplied by four lies in the same
range as the A/D converted data of the black level V.sub.SP. Under
this condition, the relation between the digital output n and input
voltage may be expressed as:
For example, when the background level V.sub.SG (n) is 103 V,
V.sub.SG (analog)=62 +102.times.39.126 mV=3.991 V; when the black
level V.sub.SP (n) is 163 V, V.sub.SP (analog)=17+162.times.9.7756
mV=1.6006 V.
We confirmed that the toner supply control employing the threshold
value V.sub.SP =4V.sub.SG, i.e., 1/4, is successful to maintain the
contrast desirably high. In this embodiment, therefore, whether or
not the toner supply is necessary is determined by comparing
digital black pattern density data attained by coupling the density
signal to the input channel A.sub.1 and digital white pattern
density data attained by coupling the density signal to the input
channel A.sub.0.
Referring again to FIG. 2, the solenoid driver 40 (FIG. 1)
comprises a switching transistor 54 the base of which is connected
to an output port P.sub.20 of the microprocessor 38. The collector
of the transistor 54 is connected to the clutch solenoid 42. When
the logical level at the output port P.sub.20 is made "1", the
transistor 54 is turned on to energize the solenoid 42 so that the
roller 44 (FIG. 1) associated with the hopper 46 is driven for
rotation. A transistor 56 is connected to the solenoid 42 in order
that a specific amount of toner supply matched to a copy size may
be supplied for each copy. Thus, the toner will also be supplied
when the transistor 56 is turned on under the control of the copy
control microprocessor 52. While at least one of the transistors 54
and 56 is turned on, that is, during toner supply, a light emitting
diode or LED 58 connected with one end of the solenoid 42 is turned
on to display the toner supply. An output port P.sub.21 of the
microprocessor 38 is connected to the base of a transistor 60 which
is in turn connected to a light emitting diode or LED 62 adapted
for monitoring purpose. At a start of A/D conversion, the
microprocessor 38 turns on the transistor 60 and thereby the
monitor LED 62 and, after a predetermined operation for setting an
amount of toner supply, it turns off the transistor 60 and thereby
the monitor LED 62.
An interrupt terminal INT of the microprocessor 38 is supplied from
the copy control microprocessor 52 with one pulse for each set of
ten copies as a toner density control command, while a power source
of the copier is turned on. An interrupt terminal T.sub.0 of the
microprocessor 38 is supplied with a train of pulses which occur
one for each predetermined small angular movement of the drum 12.
The microprocessor 38 controls the amount of toner supply by
counting the pulses synchronous with the rotation of the drum 12,
as will be described hereinafter. Further, output ports P.sub.14
-P.sub.16 and P.sub.10 -P.sub.13 of the microprocessor 38 are
connected to a connector 64 with which a monitor unit 66 will be
connected in the event of services for the copier.
The monitor unit 66 comprises a character display 68.sub.1
-68.sub.3 for displaying a white level V.sub.SG, second character
display 70.sub.1 -70.sub.3 for displaying a black level V.sub.SP,
third character display 72.sub.1, 72.sub.2 for displaying a density
ratio V.sub.SG /V.sub.SP, segment docoder 74, digit coder 76,
segment drivers 78.sub.1 -78.sub.7 and digit drivers 80.sub.1
-80.sub.8. When the monitor unit 66 is connected with the connector
64, transient values of V.sub.SG, V.sub.SP and V.sub.SG /V.sub.SP
will appear on the unit 66. The circuitry additionally includes a
toner density control command switch 82 which starts a toner
density control when closed temporarily and will be closed in the
event of services.
Referring to FIG. 4, a copy control flow of the microprocessor 52,
particularly the constant amount toner supply, will be described.
When various sections of the copier individually reach operable
conditions, the microprocessor 52 loads "1" in a copy counter
(program counter) which is adapted to provide a toner density
control command timing. In this condition, the microprocessor 52
awaits closing of a print SW. As the print SW is closed (copy
command), the charger 48 is energized to start exposure while the
pulses synchronous with the drum rotation or drum sync pulses start
to be counted. At the instant the pattern corresponding to the
white pattern P.sub.G formed on the drum 12 reaches the sensor 32,
the interrupt terminal INT of the microprocessor 38 is supplied
with a toner density control command (start pulse). While the
content of the copy counter is "2" to "10", the start pulse is not
supplied and the erase lamp 50 is energized to discharge the drum
surface over to the pattern corresponding to the black pattern
P.sub.P. Then, the copy control is continued. After one copy had
been completed, a toner supplement counter (program counter) is
loaded with sheet format data (toner supply time matched with a
sheet format and number of pulses synchronous with the drum
rotation). At the same time, the transistor 56 (FIG. 2) is turned
on. Thereafter, every time a drum sync pulse arrives, the toner
supplement counter is decremented by "1". Upon decrease of the
count to "0", the transistor 56 is turned off. Then, the copy
counter is incremented by "1". The microprocessor 52 starts another
copying cycle in a repeat copy mode but awaits closing of the print
switch SW in a single copy mode. Because the microprocessor 52
resets the copy counter to "1" each time its content reaches "11"
and delivers a toner density control command to the microprocessor
38 only when the content of the copy counter is "1", a toner
density control occurs once for ten successive copies.
The detection of toner densities of the patterns corresponding to
the white and black patterns P.sub.G and P.sub.P, ratio computation
based on the detected toner densities and setting of a toner supply
amount based on the computed ratio are commonly carried out by an
interrupt control in response to a toner density control command
pulse (start pulse) fed from the microprocessor 52 to the interrupt
input terminal INT. The control of the set amount of toner supply
and the display drive control for the displays 68.sub.1 -68.sub.3,
70.sub.1 -70.sub.3, 71.sub.1 and 71.sub.2 occur according to a main
routine.
Referring to FIG. 5a, there is shown a flowchart which demonstrates
the interrupt control. When the interrupt input terminal INT of the
microprocessor 38 changes from logical "1" to logical "0", the
microprocessor 38 makes its output port P.sub.21 logical "1" to
energize the LED 62 and loads a monitor counter (program counter)
with "16". Then, the microprocessor 38 makes its output ports
P.sub.10 -P.sub.13 and P.sub.14 -P.sub.16 logical "0" to turn off
the indication on the displays 68.sub.1 -68.sub.3, 70.sub.1
-70.sub.3, 71.sub.1 and 71.sub.2, while specifying the input
channel A.sub.0 of the A/D converter 36.
Then, the microprocessor 38 clears a read counter and awaits the
arrival of a drum sync pulse. In response to a drum sync pulse, the
microprocessor 38 reads the digitized data (8 bits) serially at its
port T.sub.1 and stores them in addition mode in an A/D data
register, by supplying data conversion timing pulses (A/D CLK) to
the A/D converter 36. After 16 (2.sup.4) times of repeated A/D
conversion and addition of the data, the content of the A/D
register is shifted 4 bits to a lower position. The resulting
content of the A/D data register indicates a mean value of the data
provided by 2.sup.4 times of A/D conversion. Because it is in
response to the arrival of the toner image corresponding to the
white pattern P.sub.G at the sensor 32 that the toner density
control command pulse (start pulse) is fed to the interrupt input
terminal INT, the digitized data associated with the specified
input channel A.sub.0 indicates a toner density of the white level
(V.sub.SG). The microprocessor 38 stores the mean value V.sub.SG of
the white level toner density in a V.sub.SG register. Then, the
microprocessor 38 clears the read counter for the detection of the
border between the white toner pattern (P.sub.G) and the black
toner pattern (P.sub.P), specifies the input channel A.sub.1 for
the A/D conversion, and performs A/D conversion in the same way in
response to a drum sync pulse. As already described, the input
channel A.sub.1 is directly supplied with a voltage indicative of a
detected toner density (without voltage division), the maximum
value of the input analog voltage is 2.5 V, the voltage (analog)
indicating a toner density of the white pattern is not lower than
2.5 V, and the voltage indicating a toner density of the black
pattern is lower than 2.5 V. For these reasons, whether the pattern
is white or black can be identified by checking whether the input
voltage at the input channel A.sub.1 is not lower than 2.5 V (the
digital data will be 2.5 V when the input voltage is not lower than
2.5 V, because the voltage is 2.5 V in full scale). Accordingly, as
long as the digital data indicates 2.5 V, the microprocessor 38
determines that the sensor 32 is still detecting the white pattern
and, so, repeats another A/D conversion. As the digital data
indicates a voltage lower than 2.5 V, the microprocessor 38
increments the read counter by "1" and carries out A/D conversion
in response to a drum sync pulse. When successive three times of
A/D conversion have shown voltages lower than 2.5 V (when the read
counter has decremented to "0"), the microprocessor 38 determines
that the toner image of the black pattern has been brought into the
detectable range of the sensor 32, specifies the input channel
A.sub.1 of the A/D converter 36 and clears the counter. Thereafter,
the microprocessor 38 waits for 5 successive drum sync pulses to
avoid detection of a transitional range. It will be noted that, if
the digitized data indicates 2.5 V even once during (m-1) times of
repeated A/D conversion after it has indicated a voltage lower than
2.5 V, the microprocessor 38 loads the read counter with "3" again
and repeats A/D conversion until the data continuously indicates
voltages lower than 2.5 V for another three times of A/D
conversion.
As the counter reaches "6", the microprocessor 38 performs A/D
conversion. Thereafter, the microprocessor carries out A/D
conversion in response to every drum sync pulse and adds 2.sup.4
times of digitized data in an A/D data register. Upon completion of
the 2.sup.4 times of conversion and summation of the data, the
microprocessor 38 shifts the content of the A/D data register by 4
bits to a lower position. The resulting content of the A/D data
register indicates a mean value V.sub.SP of the detected input
voltages (V.sub.SP). At this stage of operation, V.sub.SG shows a
value which is 1/4 the mean value of 16 times of sampling of the
white level, while V.sub.SP shows a mean value of 16 times of
sampling of the black level. Here, the microprocessor 38 compares
the densities V.sub.SP and V.sub.SG and, if the contrast is low
(V.sub.SP >V.sub.SG .fwdarw.YES), loads toner counters
(registers) 1 and 2 with a predetermined value. Because the supply
of about 1 gram of toner corresponds to a time period for which
1.792 pulses are counted, a time period to energize the solenoid 42
is expressed as k.times.1792=k.times.7.times.2.sup.8 where k is an
amount of toner supplied at a time. Therefore, the microprocessor
38 stores k.times.7.times.2.sup.8 in the toner counter (register) 1
which covers lower eight bits and the toner counter (register) 2
which covers upper eight bits. This is attained by storing "0" in
all the lower eight bits of the toner counter 1 while storing the
binary data which represents k.times.7 in the toner counter 2 for
the upper eight bits. After storing in the toner counters 1 and 2
the toner supply time (count of drum sync pulses), the
microprocessor 38 sets the output port P20 to logical "1" to
energize the solenoid 42 and, then, returns to the main routine
(FIG. 5b).
Referring to FIG. 5b which shows the main routine, the
microprocessor 38 performs a display drive control which causes the
displays 70.sub.1 -70.sub.3, 68.sub.1 -68.sub.3, 71.sub.1 and
71.sub.2 to emit light sequentially on a time sharing basis, as
long as the signal level at the port T.sub.0 (pulse synchronous
with the drum rotation) remains logical "0". As the signal at the
port T.sub.0 changes from logical "0" to logical "1", the
microprocessor 38 increments a key counter (program counter) by "1"
and energizes one display (one position). This procedure is
repeated while the signal at the port T.sub.0 remains logical "1".
After .alpha. times of repetition, the microprocessor 38 determines
that the port T.sub.0 has been logical "1" for that period and a
drum sync pulse has arrived. Then, the microprocessor 38 sets a key
end flag to logical "1" indicative of the arrival of such a pulse,
decrements the monitor counter by "1" if a flag indicative of
energization of the monitor LED 62 is logical "1", and deenergizes
the LED 62 as the content of the monitor counter decreases to "0".
As described with reference to FIG. 5a, when the microprocessor 38
is supplied with a toner density control command pulse from the
microprocessor 52, "16" is set in the monitor counter with the LED
62 turned on and the operation advances to the main routine (FIG.
5b) after the procedure from the density detection to the setting
of a toner supply time. For this reason and because the monitor
counter is decremented in response to each drum sync pulse in the
main flow of FIG. 5b, the LED 62 is turned off upon generation of
16 drum sync pulses after the completion of the interrupt procedure
shown in FIG. 5a.
After setting the key end flag to logical "1", the microprocessor
38 refers to a toner supply flag and, if it is logical "1"
indicating that the solenoid 42 has been energized, decrements the
toner counter (1, 2). As the toner counter is decremented to "0",
the microprocessor 38 turns off the solenoid 42. When the content
of the toner counter is not "0", the microprocessor 38 waits until
the signal level at the port T.sub.0 becomes logical "0" while
performing the display drive control for this period of time. In
response to a change of the signal level at the port T.sub.0 to
logical "0", the microprocessor 38 clears the key end flag and key
counter determining that one drum sync pulse has terminated and,
then, awaits a change of the signal level at the port T.sub.0 to
logical "1" while energizing the displays. In this manner, the
microprocessor 38 decrements the toner counter (1, 2) every time a
drum rotation synchronous pulse arrives; upon decrease of the
content of the toner counter to "0", that is, upon the lapse of a
toner supply time after the toner supply time has been set and the
solenoid 42 has been turned on, the microprocessor 38 turns off the
solenoid 42. Thereafter, the microprocessor 38 effects the display
drive control only.
Thus, the embodiment described above utilizes the fact that the
voltage generated by the toner image corresponding to the white
pattern area is about 4 V and that generated by the toner image
corresponding to the black pattern area is about 1.7 V which
greatly differs from 4 V, and the fact that the A/D converter 36
receives a voltage which is 2.5 V at the maximum while delivering
digital data which constantly indicates 2.5 V as long as the input
voltage is not lower than 2.5 V. After detection of the toner image
density in the white pattern area, the operation advances to the
detection of a toner image density corresponding to the black
pattern when the output data of the A/D converter 36 has indicated
a voltage lower than 2.5 V and such a voltage has continued
throughout the subsequent three times of detection, determining
that the black pattern on the drum has reached the sensor 32. This
permits the pattern density detection timing to be readily set;
only the read timing for the white or first pattern should be set.
It is thus needless to set any another timing even if the
magnification is changed. Because an amount of toner supply is
determined on the basis of the density ratio V.sub.SG between the
toner images corresponding to the white and black patterns, the
toner density control can occur relatively stably though the
characteristics of the sensor and/or drum surface may fluctuate. As
shown in FIG. 6, suppose that the output voltages of the sensor 32
sensing an image density has shifted from the standard values
indicated by a solid curve to the values indicated by a dotted
curve, due to a change in the characteristics of the sensor 32
and/or drum surface. Then, the difference between the voltages
V.sub.SG and V.sub.SP associated with the white and black toner
patterns changes from 3.9 V to 2.7 V, resulting in a change of
(3.9-2.7)/3.9.times.100=31%. Still, the ratio V.sub.SG /V.sub.SP
increases from 3.167 to 3.555 and the change is not more than
(3.555-3.167)/3.167.times.100=12.3%. Thus, the stability in toner
density control is not effected by any change in the
characteristics of the sensor and/or drum.
Furthermore, in the embodiment described, the resolution in A/D
conversion of the voltage V.sub.SP corresponding to the black toner
pattern is selected to be four times the resolution of the voltage
V.sub.SG corresponding to the white toner pattern. As well known in
the art, a developed toner image often involves white omitted spots
and black spots which result in an irregular distribution of
detected voltage levels though representing the same pattern. The
absolute value of such fluctuation is large at V.sub.SG and small
at V.sub.SP. Thus, where different resolutions are assigned to
different patterns and the input voltage levels for A/D conversion
are made substantially the same as previously described, there can
be prevented an occurrence that the fluctuation of one of the
detection levels becomes predominant relative to the other.
Particularly, in the case of A/D conversion, a common resolution
would reduce the weight of V.sub.SP relative to V.sub.SG unless
with a larger number of A/D data bits, due to the substantial range
which includes V.sub.SG and V.sub.SP. The number of A/D data bits
should be as small as possible from the viewpoint of element
construction and calculation. It will thus be apparent that
assigning different resolutions to different patterns minimizes the
number of A/D data bits and simplifies the element construction and
calculation accordingly. Additionally, whether or not to supply the
toner is determined relying not on division but on simple
comparison in magnitude, promoting simple computation as well as
fast determination.
Other embodiments and modifications of the present invention will
be described. Whether or not to supply the toner has been described
as being determined by comparing digitized data of analog voltages
V.sub.SP and V.sub.SG with respect to a threshold value of V.sub.SP
/V.sub.SG =4/1 and ranges of 1:4. Alternatively, the ranges may be
designed adjustable to make the threshold value controllable. This
is achievable in conjunction with the A/D converter 36 of FIG. 3 by
directly coupling the analog density signals to the input terminal
A.sub.1 and coupling a voltage (divided voltage) of the slider of a
10 k.OMEGA. variable resistor VR to the input terminal A.sub.0, as
illustrated in FIG. 7.
While in the embodiment shown and described the black and white
charge patterns have been formed by scanning the optical marks
MR.sub.P and MR.sub.G carried by the glass platen 10, they may be
formed through the control over energization of the charger 48,
ON/OFF control of a light source for illumination, ON/OFF control
of the erase lamp 50 or control of the bias voltage applied to the
developing roller 24.
The value associated with an image density has been shown and
described as a density of the toner image of each charge pattern
(black or white). Instead, the value concerned may be achieved by
detecting a surface potential of a charge pattern before
development or a surface potential of a toner image of the charge
pattern after development, or detecting a density of a toner image
transferred onto a sheet of paper.
Furthermore, the embodiment described has been constructed to
maintain image density constant by determining an amount of toner
supply on the basis of a ratio between the values associated with
image densities of two different patterns. Alternatively, the ratio
mentioned above may be used to control the charger 48, intensity of
illumination, bias voltage for development, toner supply to a
developer, toner supply to the developing station or transfer
potential, either independently or in combination. In any case, the
value associated with an image density differs a great deal from
one pattern to the other and fluctuates with time, temperature or
the like. Thus, for a more stable image density control, the values
corresponding to different patterns should be A/D converted with
different resolutions so that the parameter associated with image
densities may be controlled by an amount which is based on the
ratio between the digital values.
Various modifications will become possible for those skilled in the
art after receiving the teachings of the present disclosure without
departing from the scope thereof.
* * * * *